The invention relates to a process for the removal of deposits which form in a reaction apparatus by the precipitation of solids from a gaseous, liquid or liquid/gaseous reaction mixture or from a suspension.
Such deposits on reactors or other surfaces in contact with the reaction medium may consist, for example, of organic or inorganic solids which crystallise out, metallic mirrors, polymers, tarred reaction residues or other solids adsorbed at the reactor surfaces.
According to the prior art, such deposits are generally removed batchwise by mechanical and/or chemical cleaning operations, which take place as long as the reaction apparatus is not in operation (see EP 121 263 A1, WO 99/05251). As a result of such cleaning, therefore, the reaction apparatus is used less fully in terms of time, which is economically disadvantageous.
In addition, such a procedure means that the precipitates are left in place for a relatively long time. If deposits on reactor walls, pipes and built-in elements are not removed early, they can lead to disadvantageous secondary reactions during chemical reactions. Precipitates may thus age, become compacted or crystallise. The relatively long dwell time in the reactor in comparison with the reaction mixture can lead, for example by condensation, polymerisation or crosslinking, to deposits which are more difficult to remove chemically or mechanically. Organic compounds may crack or carbonise. If such deposits become detached in an uncontrolled manner, they can have an adverse effect on product properties.
The deposits may also be valuable components of a reaction mixture, such as, for example, homogeneous catalysts or their degradation products. If such expensive components cannot be discharged from the reaction apparatus with the reaction mixture, they avoid a subsequent working-up or separation step and therefore cannot be used again in the reaction in processed form.
An example of the occurrence of deposits in a chemical reaction is the process of the direct carbonylation of hydroxy aromatic compounds to diaryl carbonates. It is known that organic carbonates can be prepared by oxidative reaction of an aromatic hydroxy compound with carbon monoxide in the presence of a noble metal catalyst (DE-A-2 738 437). Palladium is preferably used as the noble metal. A co-catalyst (e.g. manganese, lead or cobalt salts), a base, a quaternary salt, various quinones or hydroquinones and drying agents can additionally be used. The operation can be carried out in a solvent such as, for example, acetonitrile, monochlorobenzene or dimethylacetamide.
In that process, catalyst components may precipitate during the reaction and be deposited on the surfaces of the reaction apparatus. It is assumed that the deposits are essentially precipitated palladium metal; the precise chemical composition of such deposits is unimportant for the process according to the invention, however. Owing to the high costs of palladium, it must be recovered from the reaction mixture. However, it is not possible to convey the palladium deposited in the reactor to a separation and processing step.
It is therefore desirable to elaborate a process which removes such deposits and allows the deposits to be discharged from the reactor with the reaction mixture.